Should I have grounded that power supply circuit at the car computer instead?
And the 12+V power supply input, should that come from somewhere else?
That's what I said.
It should come from the same place as the car computer, and the cables should run together. Obviously you can (should) use a common supply cable to both, splitting it to the corresponding two regulators at the car computer (that is, it supplies the car computer regulator and goes on to the recording computer along with all the other cables). Similarly for the ground, and as long as no other ground connection is made anywhere.
If I can figure out a nice way to splice into the ends of the wires that come into the edge connector, is that satisfactory in your mind? Or do I have to splice off the input and ground right at the power rectifier?
The AM2130 Datasheet isn't clear about whether the chip is strictly TTL or not. It says "All inputs and outputs are TTL-compatible".